Survive unpickling crashes when completing from Tasty

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -261,6 +261,7 @@ private sealed trait YSettings:
   val YdebugTreeWithId: Setting[Int] = IntSetting("-Ydebug-tree-with-id", "Print the stack trace when the tree with the given id is created.", Int.MinValue)
   val YdebugTypeError: Setting[Boolean] = BooleanSetting("-Ydebug-type-error", "Print the stack trace when a TypeError is caught", false)
   val YdebugError: Setting[Boolean] = BooleanSetting("-Ydebug-error", "Print the stack trace when any error is caught.", false)
+  val YdebugUnpickling: Setting[Boolean] = BooleanSetting("-Ydebug-unpickling", "Print the stack trace when an error occurs when reading Tasty.", false)
   val YtermConflict: Setting[String] = ChoiceSetting("-Yresolve-term-conflict", "strategy", "Resolve term conflicts", List("package", "object", "error"), "error")
   val Ylog: Setting[List[String]] = PhasesSetting("-Ylog", "Log operations during")
   val YlogClasspath: Setting[Boolean] = BooleanSetting("-Ylog-classpath", "Output information about what classpath is being applied.")

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -115,10 +115,25 @@ class TreeUnpickler(reader: TastyReader,
     val owner = ctx.owner
     val source = ctx.source
     def complete(denot: SymDenotation)(using Context): Unit =
-      treeAtAddr(currentAddr) = atPhaseBeforeTransforms {
-        new TreeReader(reader).readIndexedDef()(
-          using ctx.withOwner(owner).withSource(source))
-      }
+      def fail(ex: Throwable) =
+        def where =
+          val f = denot.symbol.associatedFile
+          if f == null then "" else s" in $f"
+        if ctx.settings.YdebugUnpickling.value then throw ex
+        else throw TypeError(
+          em"""Could not read definition of $denot$where
+              |An exception was encountered:
+              |  $ex
+              |Run with -Ydebug-unpickling to see full stack trace.""")
+      treeAtAddr(currentAddr) =
+        try
+          atPhaseBeforeTransforms {
+            new TreeReader(reader).readIndexedDef()(
+              using ctx.withOwner(owner).withSource(source))
+          }
+        catch
+          case ex: AssertionError => fail(ex)
+          case ex: Exception => fail(ex)
   }
 
   class TreeReader(val reader: TastyReader) {

tests/neg/i12482.check

@@ -0,0 +1,6 @@
+-- [E006] Not Found Error: tests/neg/i12482.scala:3:2 ------------------------------------------------------------------
+3 |  FileZipArchive // error
+  |  ^^^^^^^^^^^^^^
+  |  Not found: FileZipArchive
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg/i12482.scala

@@ -0,0 +1,4 @@
+package dotty.tools.io
+object ZipArchive {
+  FileZipArchive // error
+}